The invention relates to a device for determining the rotational speed of a transfer element arranged between two sets of variable-diameter pulleys in the transmission case of a continuously variable transmission system in accordance with the preamble contained in patent claim 1.
Continuously variable transmissions, typically of the type described in DE-OS 42 27 958.8, are equipped with two sets of variable-diameter pulleys, between which a transfer element, such as a chain or steel belt, is arranged. This transfer element serves to transmit the propulsive power generated by the internal combustion engine of a motor vehicle from an input shaft to an output shaft. In order to vary the transmission ratio, and thus the drive torque reaching the vehicle's wheels, it is usual for one variable-diameter pulley of each set of pulleys to be capable of hydraulic displacement on the input or output shaft of the transmission case. As a result of variations in the distance between the pulleys, variations in the effective transfer radii of the transfer element are produced in the respective sets of variable-diameter pulleys. The amount of pressure which is generated between the respective variable-diameter pulleys at a preset distance, and applied to the transfer element for the transmission of power, is dependent on the peripheral force acting on the transfer element and is automatically adjusted by a transmission control system, it usually being necessary to make provision for conditions of uncertainty by clear surplus allowances.
To achieve a satisfactory degree of overall efficiency, or to achieve the minimum loss of power from a transmission system of this type, it is necessary to minimise this applied pressure as far as possible. In addition, provided the pressure required to be applied by the pulleys to the transfer element at any given time is precisely known, the maximum power required from the hydraulic pump for the pulley displacement system can be reduced, thereby not only saving space in the transmission case but also costs.